Bed. Asaorticpressurewasheldconstant,thecardiacoutput,heart rate,andcoronaryarteryperfusionpressurewereabruptlyincreased ordecreasedfromthecontrollevel,whichisindicatedbythepointatwhichthetwolinescross.Thesolid circles representtheflowsthat resultedimmediatelyafterthechangeinperfusionpressure;theopen circles representthesteady-stateflowsatthenewpressures.Thereisatendencyforflowtoreturntowardthecontrollevel(autoregulationofbloodflow),andthisismostprominentovertheintermediatepressurerange(≈60to180mmHg).(FromBerneRM,RubioR.Coronarycirculation.InPageE,ed.Handbook of Physiology: Section 2: The Cardiovascular System: The Heart. Vol1.Bethesda,MD:AmericanPhysiologicalSociety;1979.) limits by baroreceptor reflex mechanisms. Hence, changes in coronary blood flow are caused mainly by changes in the diameter of coronary resistance vessels in response to the metabolic demands of the heart.
•Fig. 17.31 Comparison of Phasic Coronary Blood Flow in the Left and Right Coronary Arteries. Extravascularcompressionissogreatduringearlyventricularsystolethatthedirectionofbloodflowinthelargecoronaryarteriessupplyingtheleftventricleisbrieflyreversed.Maximalinflowintheleftcoronaryarteryoccursinearlydiastole,whentheventricleshaverelaxedandextravascularcompressionofthecoronaryvesselsisvirtuallyabsent.Afteraninitialreversalinearlysystole,bloodflowintheleftcoronaryarteryfollowstheaorticpressureuntilearlydiastole,whenitrisesabruptlyandthendeclinesslowlyasaorticpressurefallsduringtheremainderofdiastole.
In addition to providing the pressure to move blood through the coronary vessels, the heart also affects its blood supply by the squeezing effect (extravascular compression) of the contracting myocardium on its own blood vessels. The patterns of flow in the left and right coronary arteries are shown in
Fig. 17.31 . In the left ventricle, coronary perfusion pressure is the difference between aortic diastolic pressure and left ventricular end-diastolic pressure.
Left ventricular myocardial pressure (pressure within the wall of the left ventricle) is highest near the endocardium and lowest near the epicardium. This pressure gradient does not normally impair endocardial blood flow because the greater blood flow to the endocardium during diastole compensates for the greater blood flow to the epicardium during systole. Measurements of coronary blood flow indicate that the epicardial and endocardial halves of the left ventricle receive approximately equal blood flow under normal conditions. Because extravascular compression is greatest at the endocardial surface of the ventricle, the equality of epicardial and endocardial blood flow indicates that the tone of the endocardial resistance vessels is less than that of the epicardial vessels.
Theminimalextravascularresistanceandabsenceofleftventricularworkduringdiastolecanbeusedtoimprovemyocardialperfusioninpatientswithadamagedmyocardiumandlowbloodpressure.Inamethodcalledcounterpulsation, aninflatableballoonisinsertedintothethoracicaortathroughafemoralartery.Theballoonisinflatedduringeachventriculardiastoleanddeflatedduringeachsystole.Thisprocedureenhancescoronarybloodflowduringdiastolebyraisingdiastolicpressureatatimewhencoronaryextravascularresistanceislowest.Furthermore,itreducescardiacenergyrequirementsbyloweringaorticpressure(afterload)duringventricularejection.
The flow pattern in the right coronary artery is similar to that in the left coronary artery (see